Since their discovery, controlled radical polymerization (CRP) processes have gained increasing attention because CRP couples the advantages afforded by conventional free radical polymerization (RP), to (co)polymerize a wide range of monomers using various commercially viable processes, with the ability to synthesize polymeric materials with predetermined molecular weight (MW), low polydispersity (Mw/Mn), controlled composition, site specific incorporated predetermined functionality, selected chain topology and the ability to incorporate bioresponsive or inorganic species into the final product.
Atom transfer radical polymerization (ATRP) is considered to be one of the most successful CRP processes with significant commercial potential for production of many specialty materials including coatings, sealants, adhesives, dispersants in addition to materials for health and beauty products, electronics and biomedical applications. The process, catalysts, including transition metals and ligands, range of polymerizable monomers and materials prepared by the process have been thoroughly described in a series of co-assigned U.S patents and Applications including U.S. Pat. Nos. 5,763,548; 5,807,937; 5,789,487; 5,945,491; 6,111,022; 6,121,371; 6,124,411; 6,162,882; 6,407,187; 6,512,060; 6,538,091; 6,541,580; 6,624,262; 6,624,263; 6,627,314; 6,759,491; 6,790,919; 6,887,962; 7,019,082; 7,049,373; 7,064,166; 7,125,938; 7,157,530; 7,332,550; 7,572,874; 7,678,869; 7,795,355; 7,825,199; 7,893,173; 7,893,174 and U.S. patent applications Ser. Nos. 10/548,354; 11/990,836; 12/311,673; 12/451,581; 12/921,296; 12/877,589; 12/949,466 and 13/026,919 the disclosures of all of which are herein incorporated by reference. These prior art patents and applications describe the range of (co)polymerizable monomers in addition to the topology, architecture and site specific functionality attainable in copolymers prepared by ATRP in addition to detailing a range of composite structures that can be prepared by “grafting from” or “grading to” a broad range of organic or inorganic materials.
ATRP has also been discussed in numerous publications with Matyjaszewski as co-author and reviewed in several book chapters. [Matyjaszewski, K. et al. ACS Symp. Ser. 1998, 685, 258-283; ACS Symp. Ser. 1998, 713, 96-112; ACS Symp. Ser. 2000, 729, 270-283; ACS Symp. Ser. 2000, 765, 52-71; ACS Symp. Ser. 2000, 768, 2-26; ACS Symposium Series 2003, 854, 2-9; ACS Symp. Ser. 2009, 1023, 3-13 and Chem. Rev. 2001, 101, 2921-2990.] These publications are incorporated by reference to provide information on the range of suitable transition metals that can participate in the redox reaction and suitable ligands for the different transition metals to form transition metal complexes suitable for polymerizing broad range of exemplified polymcrizable (co)monomers. The generally accepted mechanism of an ATRP reaction is shown in Scheme 1.

Any transition metal complex (Mtn/L) capable of maintaining the dynamic equilibrium through participation in a redox reaction comprising the transferable atom or group present on each initiator or dormant polymer chain (Pn—X) to form an active radical (Pn•) and higher oxidation state transition metal complex (X-Mtn+1/L) that acts as the deactivator, may be used as the catalyst in ATRP. The creation and maintenance of a low concentration of active species, (Pn•), reduces the probability of bimolecular termination reactions, (kl), which leads to a radical polymerization process that behaves as a “living” system through retention of the stable transferable atom or group (—X) on the vast majority of growing dormant chain ends. The most frequently used ATRP procedure is based on a simple reversible halogen atom transfer catalyzed by redox active transition metal compounds, most frequently copper or iron, that form a catalyst complex with a ligand that modifies solubility and activity of the catalyst, most frequently nitrogen based ligands. The simple procedure may be carried out in bulk, in the presence of organic solvents or in water, under homogeneous or heterogeneous conditions, in ionic liquids, and in supercritical CO2.
Early ATRP procedures required addition of a sufficiently high concentration of the transition metal complex to overcome the effect of unavoidable increased concentration of the deactivator in the reaction medium while still driving the reaction to the desired degree of polymerization in a reasonable time frame while retaining chain end functionality. Recently a novel approach that allowed a significant reduction in the concentration of added catalyst was developed. [PCT Int. Appl. WO 2005/087819; Camegie Mellon University, 2005; 96 pp.] The driving force was the economic penalty associated with purification procedures coupled with a deeper understanding of the ATRP rate law (Equation 1. using CuI as the catalyst metal) which shows that Rp, the polymerization rate, depends only on the ratio of the concentration of [CuI] to [X—CuII], and does NOT depend on the absolute concentration of the copper complexes (Equation 1). Therefore in principle, one could reduce the absolute amount of copper complex to ppm levels without affecting the polymerization rate.
                              R          p                =                                                            k                p                            ⁡                              [                M                ]                                      ⁡                          [                              P                *                            ]                                =                                                    k                p                            ⁡                              [                M                ]                                      ⁢                                                            K                  eq                                ⁡                                  [                  I                  ]                                            o                        ⁢                                          [                                  Cu                  I                                ]                                            [                                  X                  -                                      Cu                    II                                                  ]                                                                        (        1        )            
However, a balance had to be reached between the formed activator species (i.e. CuI/L, where L=ligand) and a residual amount of deactivating species (i.e. X—CuI/L) which is required for a well-controlled polymerization since both, molecular weight distribution and initial molecular weight, depend on the ratio of the propagation and deactivation rate constants and the concentration of deactivator, formula (2).
                                          M            w                                M            n                          =                  1          +                      1                          DP              n                                +                                    (                                                                                          [                                              R                        -                        X                                            ]                                        o                                    ⁢                                      k                    p                                                                                        k                    da                                    ⁡                                      [                                          X                      -                                              Cu                        II                                                              ]                                                              )                        ⁢                          (                                                2                  q                                -                1                            )                                                          (        2        )            
This means, for example, that in order to obtain polystyrene with Mw/Mn=˜1.2, when targeting a DP˜200 and 90% conversion at ˜100° C, the actual amount of X—CuII species required to conduct a controlled reaction is ˜2 ppm (kp˜103 M−1s−1 and kds˜107 M−1s−1), meaning that the concentration of the X—CuII species could be reduced over 1,000 times from the level typically used in the earlier ATRP polymerization protocols. Unfortunately, if the amount of CuI is reduced 1,000 fold, unavoidable radical-radical termination reactions irreversibly consume the activators present in the reaction media as the polymerization progresses and the reaction slows down or stops; i.e. if ˜10% of chains terminate and the amount of CuI initially added to the system was below 10 mole % of the initiator, all CuI would be consumed by termination. It was recognized that this situation could be overcome if there was constant regeneration of the CuI activator species by environmentally acceptable organic or inorganic reducing agents to compensate for any loss of CuI by termination, Scheme 2.

This procedure was named Activator ReGenerated by Electron Transfer (ARGET) ATRP [Macromnlecules 2006, 39, 39-45.] and it was possible to use a range of reducing agents; e.g. tinII-2-ethylhexanoate, ascorbic acid, glucose, amines, excess ligand, and Cu0 etc. for ARGET or a source of free radicals, such as AIBN, for Initiators for Continuous Activator Regeneration (ICAR) to constantly regenerate the ATRP activator, exemplified by a CuI species, from the deactivator, the CuII species in Scheme 2, formed during termination processes.
The electrochemical mediated ATRP procedure disclosed herein overcomes the limitations of ARGET and ICAR ATRP in that no undesirable byproducts are formed and a ratio of CuIL:CuIIL can be selected and retained or adjusted throughout the polymerization.
Cyclic voltammetry (CV) has been used for over a decade as an analytical tool to study the redox behavior of numerous transition metal complexes used in an ATRP. One of the earliest studies, [Qiu, J., et al., Macromol. Chem. Phys. 2000, 201, 1625-1631.] determined that the half-sum of the oxidation and reduction peak, the E1/2 value, strongly depends on the nature of the ligand and the halogen and the measured value provided an estimate for the activity of the catalyst complex (CuIL/CIIL redox couple) in an ATRP, and that this value strongly depends on the nature of the ligand (L) and the halogen. The general trend agreed with the kinetic features of ATRP catalyzed by the corresponding transition metal complex, and a correlation between the measured redox potential and the apparent equilibrium constant of ATRP was observed. The more negative the redox potential of the complex, as measured by CV, the faster the polymerization indicating that, in most cases, the catalytic activity of the transition metal complexes in an ATRP can be predicted from the redox potential of the transition metal complex. Two more recent studies by the primary author, K. Matyjaszewski, on a broader spectrum of transition metal/ligand complexes in a number of different solvents confirmed the conclusion that excellent correlation existed between the equilibrium constants with the CuII/CuI redox potential and the carbon-halogen bond dissociation energies. [Matyjaszewski; et al. Macromoleades 2007, 40, 8576-8585 and J. Am. Chem. Soc. 2008, 130, 10702-10713.]
This analytic tool, CV analysis, was used extensively to provide information on the expected properties of the transition metal complex in an ATRP. [Bortolamei, N., et al., Macromolecules, 2010: 43, 9257-67 and Electrochim. Acta, 2010, 55(27): 8312-8318.] CV analysis had always been carried out in the absence of monomer and in the absence of initiator. However, there is a recent paper, by one of the authors of the initial CV paper, (C. Amatore) where the notion of utilizing an electrochemical technique to produce an electrogenerated FeIISalen complex providing activation of alkyl- and benzyl halide initiators as the initial step of an atom transfer radical addition (ATRA) reaction. [J. Electroanal. Chem. 2009, 633, 99-105] CV's conducted in the absence and presence of an ATRP initiator showed that reductive cleavage of the R—X bond occurred on the timescale of the CV measurement but suggested that it does not lead to a classical redox-catalysis framework. Additionally, it was noted that addition of a monomer adversely affected the voltammogram of the studied iron complex. Attempts to polymerize styrene at 110° C., a temperature at which self initiated polymerization occurs, in the presence of a FeIISalen complex formed by electrolysis resulted in the formation of low molecular weight oligomers with broad polydispersity over a three hour period (MW 1868, Mw/Mn1.768). Furthermore, the paper indicates that multiple transition metal/ligand/initiator species were involved in the reaction and determined that activation of alkyl halides by an electrogenerated FeIISalen complex did not proceed along the redox mediated process usually invoked in an ATRP and the results presented showed no evidence of the development of a CRP process. These results would teach against using electrochemistry to improve the degree of control an ATRP.
In US2011/0034625A1 the concept of using electrochemically produced free radicals for the initiation of a standard free radical polymerization is considered. The procedure focuses on direct formation of free radicals, substantially hydroxyl radicals, on the basis of electrolysis at anodic electrode surfaces. While indicating that the procedure could be employed to initiate a CRP this reference provides no evidence of a methodology which would allow manipulation of a redox-active species via electrochemical methods to subsequently control a CRP procedure.
There are no reports in the decade since the first report using cyclic voltammetry as an analytical tool to measure the redox potential of a transition metal complex to determine if electrochemistry could be used to mediate an ATRP. The notion of utilizing an electrochemical technique (i.e. electrolysis) to modulate polymerization kinetics has never been reported.